1. Field of the Invention
The present disclosure relates to vehicular traffic control indicators. More particularly, the invention relates to parasitic power supplies suitable for use with traffic control indicators.
2. Description of Related Art
In the field of traffic control, it is desirable to incorporate new devices, including cameras, speed detection systems, and various sensors for traffic or other security monitoring tasks. Traffic signal lights are centrally located and accessible, and have a supply of electrical power, so it would be desirable to make use of this via a parasitic power supply so that any retrofitted device could be powered without running a completely new power line from the grid to the new device.
Unfortunately, drawing power in parallel with traffic signal lamps presents a significant issue of safety because the operation of the lamp and of the lamp integrity sensing circuits must not be compromised. It is imperative that no two lamps are tied together through any single-point or credible fault of the parasitic power module. Furthermore, it may be necessary in some systems to let the traffic lamp power up normally for some period of time, usually less than one second, so that lamp current monitors will not be confused.
U.S. Pat. No. 5,736,795 to Zuehlke et al. describes how to steal power in a “series” switch, like a light switch, where no neutral is available. It uses a single source of power, and steals portions of AC cycles from the load. This approach is not suitable for use in traffic controls because it derives from a single source, not a commutating load, and it steals “series” power, whereas in traffic signals one needs to steal “parallel” power. Furthermore, only small amounts of power can be stolen, and it affects the load. The focus is to steal power for the device controlling the load, not for other applications.
U.S. Pat. No. 5,811,963 to Elwell is a similar approach to that of Zuehlke et al. '795 but power stealing is just serial. Only small amounts of power can be gotten this way, and it affects the load. U.S. Pat. No. 5,903,139 “Power stealing solid state switch for supplying operating power to an electronic control device” to Kompelien describes a methodology similar to that of Zuehlke et al. '795.
U.S. Pat. No. 6,356,038 to Bishel describes an AC controller that provides programmable switching of AC power flow, together with producing a source of DC power for operating the AC controller.
U.S. Pat. No. 6,657,418 to Atherton describes a parasitic power supply. As in several of the aforedescribed patents, the focus is serial power stealing, for powering the load controller.
U.S. Pat. No. 7,142,813 to Desai describes a system in which power is pulled from the digital data stream. This technique has been in use since the 1980s or '90s to steal power from serial and parallel ports of computers, to power small devices. U.S. Pat. No. 7,291,938 to Phinney describes a parasitic power supply intended to extract power from signals that are inbound or outbound from a piece of equipment. Neither of these patents describes a technique that can be used to pull more than “milliwatts” of power.
U.S. Pat. App. Pubs. 20080013351 and 20080031019 to Alexander describe methods for transforming electric power between two or more portals.